1. Field of the Invention
The present invention relates to a sheet processing apparatus for correcting skews and performing punching processes on the sheets delivered from an image forming apparatus such as an MFP (multi function peripherals), which is a composite digital machine, a copying machine or a printer.
2. Description of the Related Art
Image forming apparatus such as MFPs, copying machines and printers are provided at a sheet delivery section of the image forming apparatus main body with a post-processing unit for performing post-processing operations including an operation of sorting sheets where an image is formed, that of stapling sheets and/or that of punching sheets.
In such a post-processing unit, the sheet of paper delivered from the image forming apparatus can move aslant and become skewed relative to the sheet delivering direction. When the skewed sheet of paper is subjected to a punching process (and punched), one or more than one holes can be produced at wrong positions by the punching process to give rise to a problem for a succeeding filing operation. For this reason, the post-processing unit is equipped with a skew correcting means for correcting the skew (if any) of the sheet of paper before the punching process.
Jpn. Pat. Appln. Laid-Open Publication No. 2002-274699 describes a sheet processing apparatus that can correct skews. The apparatus described in the above-cited Patent Document comprises a punch unit for performing a punching process on the sheet of paper being delivered to it and the sheet of paper is made to abut a nipper of an inlet roller arranged upstream of the punch unit in order to correct the skew, if any, of the sheet of paper. However, with the apparatus described in the above-cited Patent Document, the sheet of paper that has been corrected for the skew is forced to travel a long distance before it gets to the punch unit. Therefore, the sheet of paper can be skewed again during the travel over that distance.
Jpn. Pat. Appln. Laid-Open Publication No. 2002-193493 described a punching processing apparatus that can correct the skew, if any, of a sheet of paper. In the punching processing apparatus described in the above-cited Patent Document, a registration member (registration roller) is arranged downstream relative to the punching means of the apparatus and the front end of the sheet of paper being delivered from an image forming apparatus main body is made to abut the registration member by the sheet conveying force of the sheet delivery roller of the image forming apparatus main body in order to correct the skew, if any.
However, since punching processing apparatus described in the above-cited Patent Document utilizes the sheet conveying force of the sheet delivery roller of the image forming apparatus main body, a certain degree of positional accuracy is required for aligning the image forming apparatus main body and the post processing apparatus to by turn poses limitations to the mechanical configuration of the post processing apparatus.
In view of the above-identified circumstances, it is therefore an object of the present invention to provide a sheet processing apparatus having a simple configuration that can correct the skew, if any, of a sheet of paper and accurately execute a punching process.